1. Field of the Invention
This invention pertains to an electrophotographic image recording forming apparatus and method and, more particularly, to an image-recording apparatus and method for exposing an image-carrying member by electro-optic imaging devices.
2. Description Relative to the Prior Art
Apparatus for selectively removing electrostatic charge on an image-carrying member using LEDs or lasers in electrophotographic copiers or printers is well known.
In high speed LED (light-emitting diode) electrophotographic copiers or printers, several thousand LEDs are typically arranged in a row for recording on a suitable photoconductive web or drum. The overall length of all the LED arrays is as long as the width of the photoconductive web or dram. Driver circuitry are provided for selectively activating the LEDs to emit light to record in accordance with electronic data signals. In grey level LED printers such as disclosed in PCT Publication WO 91/10311 and U.S. Pat. No. 5,200,765 (the contents of both of which are incorporated herein by this reference), the data signals may be multibit digital signals for determining an exposure duration for recording each pixel. As also noted in the aforementioned publication and patent, the LEDs are known to be non-uniform light emitters relative to each other and correction is therefore desirable to overcome image degradation due to non-uniformities between the LEDs. One form of correction is adjustment Of pulsewidth duration for recording each pixel so that any two LEDs on the printhead, when each is enabled to record a latent image of a pixel of a desired density, will provide approximately identical densities even though the light output (intensity) from each is very different. The latent image is created on the photoconductor and then developed using conventional electrophotographic processes.
In addition to errors in LED non-uniformity correction, there are similar artifacts caused by improper placement of LEDs during assembly of the LEDs on the printhead. In assembly of the printheads, arrays of LEDs on semiconductor chips are positioned in a row. A row of plural LEDs on any one chip array will be uniformly spaced (pitched) apart at say 400 LEDs to the inch (15.75 per mm) due to the accuracy of the manufacturing process of such chips. Photolithography is typically used to fabricate the individual elements in an LED chip array. The placement of the individual LEDs within an array can thus be done with sub-micron accuracy; however, the semiconducting wafers used to produce LED chip arrays are available only in sizes of three inches (7.62 cm) in diameter or smaller. It is thus not possible to fabricate a one-piece LED array long enough to be used in printer/copiers, which typically have photoconductors that are eleven inches (27.94 cm) or more in width. Thus, LED array writers are fabricated by assembling shorter LED arrays, typically about a half-inch (27.94 cm) in length, into a twelve-inch (30.48 cm) or longer array by precision mechanical placement and bonding techniques.
The mechanical placement techniques cannot hold the same sub-micron tolerances that photolithography allows. The assembled LED array writer (see FIG. 1 ) thus typically consists of half-inch (1.27 cm) segments or chip arrays, A, with individual LEDs, L, precisely aligned with respect to each other, but the cumulative placement errors of the segments combine across the width on the writer with the result that some LEDs are displaced from a straight line connecting the two end elements by a substantial portion of the pixel width.
Another source of bow, which is often greater than that caused by the LED array placement errors, is created by nonuniformities in the graded index lens arrays, such as Selfoc lens arrays, commonly used to image the LED arrays onto the photoconductor. Such lens arrays are fabricated by bonding individual graded index optical fibers into an array, cutting the fibers to the appropriate length, and then polishing the ends. Because the index gradients of the individual fibers vary somewhat and the axes of the individual fibers are never perfectly aligned with respect to each other, the images of the LEDs on the photoconductor are typically displaced somewhat from each other. This optical source of bow thus results in misalignment of the images of the LEDs even within LED array segments. Thus, the images of pixels from the LEDs after focusing by a lens to the extent that such images depart from one straight line, are referred herein as "bow."
When only one writer is used in a printer, bow of a few pixel widths is generally not detectable; however, when two or more writers are used to expose the latent image for the same page, such as done for single pass or single frame color separations for color printing or for accent color, the bow can cause misregistration between the images written by the separate writers that is noticeable. Current best practice is to assemble the writers to as close a tolerance as can be justified by cost, and accept the misregistration that results. This misregistration is often several pixel heights in extent.